1. Field of the Invention
The present invention relates generally to a method of compensating for the distorted secondary current of a current transformer and, more particularly, to a method of compensating for distorted secondary current in the case where secondary current is distorted due to saturation caused by the magnetic characteristics of a steel core current transformer.
2. Description of the Related Art
A current transformer is a device for reducing the magnitude of current and inputting it to a protective relay. In order to maximize the linkage of primary and secondary windings, a steel core current transformer is generally used.
FIG. 1 is a diagram showing the configuration of a typical current transformer. Referring to FIG. 1, the current transformer includes a steel core 14 for condensing magnetic flux generated by primary current flowing through an electrical line 12, and a secondary coil 16 surrounding the steel core 14 to generate secondary current using the magnetic flux induced to the steel core 14.
FIG. 2 is a schematic diagram showing the equivalent circuit of the typical current transformer of FIG. 1. In FIG. 2, L is the magnetizing inductance of the current transformer, im is magnetizing current, i2′ is secondary current in conformity with a current transformation ratio, and i2 is secondary current actually measured. In this case, the magnetizing inductance L is not a constant value, but is a value that varies according to current. When magnetic flux increases and exceeds a specific limit, magnetizing inductance varies significantly, which is attributable to the variation in the internal state of a current transformer. In this case, it is stated that the current transformer has been saturated.
Since the magnitude of magnetizing current im is small when a current transformer operates normally, the measured secondary current value of the current transformer is proportional to primary current value, so that there is no problem. However, when the current transformer is saturated and the magnetizing inductance value of the current transformer varies significantly, the secondary current value of the current transformer varies significantly. That is, when the current transformer is saturated, the value L decreases significantly and the magnetizing current im increases, i2′ and i2 become different from each other. Accordingly, before and after the saturation of the current transformer, the relationship between finally measured secondary current i2 and primary current varies, so that the saturation of the current transformer delays the operational time of a relay and causes the mal-operation of the relay.
As described above, the steel core current transformer cannot avoid saturation due to the magnetic characteristics thereof. In this case, the secondary current passed through the current transformer is distorted, so that it is impossible to obtain information on the primary side of a system. As a result, the saturation of the current transformer causes the mal-operation or non-operation of a relay, reduces the sensitivity of a relay and delays the operational time of a relay, so that countermeasures against this problem should be provided.
A typical method for minimizing the influence of the saturation of a current transformer on a relay is to employ a current transformer having a voltage rating that is two times higher than the maximum fault current. However, even when the method is employed, there is a possibility of saturation due to the influence of a current offset component and remanent magnetic flux in the steel core. Accordingly, when the expected fault current is large, the cross section of the steel core should be large, so that disadvantages arise in that the size and cost of a current transformer increase.
For the countermeasures against the saturation of a current transformer, research into methods of compensating for secondary current distorted due to the saturation of the current transformer has been conducted.
One of the proposed methods is a method of estimating precise secondary current by estimating remanent magnetic flux and calculating magnetizing current using the magnetization curve of a current transformer. However, this method is disadvantageous in that error becomes larger if remanent magnetic flux exists at the time of the occurrence of a fault because remanent magnetic flux in an early stage is assumed to be zero.
Another of the proposed methods is a method of increasing the precision of a current transformer by estimating the magnetic flux of a steel core in an early stage and estimating exciting current using a hysteresis curve. However, this method is disadvantageous in that it is effective only in the case where the primary current of a current transformer has a sine waveform that does not include a Direct Current (DC) offset component, and it requires precise current transformer magnetization curve data.